A static memory is a device in which the stored data is retained at all times during which power is applied to the memory device. In a random-access memory (RAM), data words are stored in locations which may be independently accessed.
One method of storing data is to use two inverters connected such that the output of the first inverter is coupled to the input of the second inverter and the output of the second is coupled to the input of the first inverter. Here, if the output of the first inverter, and therefore the input of the second inverter, is a logical "1", the output of the second inverter is a logical "0". This node is also of the input of the first inverter and therefore maintains the output of the first inverter at a logical "1".
As computer systems become faster and more complex there is a continuing desire to have memory cells which are faster and smaller. If each individual cell is smaller, more cells may be formed on a given chip which in turn lowers systems costs and increases design flexibility. In addition, as cells get smaller the operation is faster.
Also, as dimensions are decreased, memory cells become increasingly sensitive to upset by energetic particles such as cosmic rays in space or alpha particles emitted from common packaging material. One approach to decrease the sensitivity of SRAM cells to upset is to add resistance to the cross-coupling of the two inverters. However, this approach can increase the size of the cell.
Accordingly, improvements in the overall size of static memory cells, particularly with improved resistance to upset, are presently desirable.